planktonic foraminifera biozonation of the middle eocene

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Berita Sedimentologi BIOSTRATIGRAPHY OF SE ASIA – PART 3

Number 31 – November 2014

Planktonic foraminifera biozonation of the Middle Eocene-

Oligocene Kebo Formation, Kalinampu area, Bayat, Klaten,

Central Java

Dian Novita, Didit Hadi Barianto and Moch. Indra Novian

Dept. of Geological Engineering, Gadjah Mada University, Yogyakarta

ABSTRACT

Research on foraminifera from the Paleogene volcanoclastics-dominated Kebo Butak Formation of Central Java is limited. A study was conducted in the Kalinampu and surrounding areas of Bayat, Klaten, Central Java. The study included measuring of three stratigraphic sections and geological mapping. The study area contains the Nampurejo pillow lava, which is considered to be the base of the Kebo-Butak Formation (part of the 'Old Andesites' complex of South Java), but its Middle Eocene age is older than previously assumed. Correlation and biozone interpretation allowed the recognition of 12 planktonic foraminifera zones, ranging in age from Middle Eocene (P11) to Early Miocene (N5). Depositional environments are all deep marine, ranging from lower bathyal to upper bathyal.

INTRODUCTION

The purpose of this study was to determine the Eocene-Oligocene planktonic foraminifera

biozonation and age of the lower part of Kebo

Formation in the Kalinampu, Sendangrejo and

Mojosari areas, Bayat district, Klaten,

approximately 40 kilometers SE of the city of

Yogyakarta (Figure 1). The Kebo formation is dominated by volcanoclastic sediments, deposited

in a marine environment, and is important

because it demonstrates a period of early

volcanism in the Southern Mountains of Java. The

thick volcanoclastic-rich section is underlain by a relatively thin series with white marls that are rich

in Middle Eocene - Early Oligocene deep marine

planktonics-dominated faunas, and which are the

focus of this study.

The post-Kebo-Butak Miocene planktonic foraminifera biozonation in the Southern

Mountains of Central Java was well documented

by Kadar (1986), but research on Paleogene

foraminifera near the base of the Kebo Formation

has been very limited. Sumarso and Ismoyowati (1975) were the first to mention the presence of

Middle and Late Eocene (P14-P15) planktonic

foraminifera in the Wungkal-Gamping Formation

of the nearby Jiwo hills, but they did not provide

any details on faunal succession and localities.

The other area in Central Java from which similar Eocene planktonic foraminifera have been reported

is the Nanggulan area, West of Yogyakarta

(Hartono 1969; Siregar and Harsono 1981 and an

unpublished study by Lunt and Sugiatno, 2003).

This paper reports the results of our study on

small foraminifera in the Kalinampu, Sendangrejo,

Mojosari and Mranggen areas (Figure 1). The

outcrop sections selected in the area are considered to represent the basal part of the Kebo

Formation, because they are located closest to the

Nampurejo pillow lava, which is considered to be

the base of the formation. While there are some

complexities such as faults, it is still relatively easy

to reconstruct a continuous rock sequence.

REGIONAL GEOLOGY AND STRATIGRAPHY The study area is part of the Southern Mountains

of Central Java, at the northern margin of what

Van Bemmelen (1949) called the Baturagung

Range. According to Toha et al (1994) the Cenozoic

sediments of the Southern Mountains were mainly

formed by gravity depositional processes, and are about 4000 meters thick. Almost the entire section

is tilted to the south due to tectonic forces active

since the Late Oligocene to Late Miocene.

Basement of the basin is composed of pre-Tertiary metamorphic rocks. According to Sumosusastro

(1956) these rocks are composed of phyllite, mica

schist, gneiss and crystalline limestones.

Metamorphic basement is overlain by the Eocene

Wungkal-Gamping Formation, which is composed

of Eocene sandstone, sandy marl, mudstone, claystone and limestone lenses. Overlying the

Wungkal-Gamping Formation are the Kebo-Butak,

Semilir and Nglanggran Formations, all are

composed of volcanoclastic sediments. They

represent a period of intensive volcanic activity in

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the Southern Mountains, and were referred to as 'Old Andesites' by Van Bemmelen (1949).

Figure 1. Location map of study area SE of Yogyakarta, showing three lines of measured sections.

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Our study is focused mainly on the lower part of

Kebo Formation. A comparison of Southern

Mountains stratigraphic columns from several

researchers shows that different authors assigned different ages to the Kebo Formation (Figure 2). Toha et al. (1994) and Surono (2009) placed the

age of the base of the Kebo-Butak Formation in the

Early Oligocene (N2).

The Kebo-Butak Formation was deposited in a marine basin surrounded by active volcanoes. The

volcanoes became major sediment suppliers to the

nearby basin. The oldest volcanic deposit is the

Nampurejo pillow lava (Figure 4). According to

Surono (2008) the age of the Nampurejo pillow lava is between ~33.2 to 31.3 Ma (Early Oligocene),

older than the age of most of the volcanoclastic

Kebo-Butak Formation, which generally ranges

between ~26.5 and 21 Ma (Late Oligocene - Early

Miocene). Deposition of the Kebo-Butak formation

indicated widespread arc volcanism in the

Southern Mountains.

Geology of the Kalinampu Area Geological mapping was conducted to determine

the lateral distribution of the rocks. The oldest

fossiliferous rock is the mudrock micrite layer that

is intercalated between volcanic breccias (Figure 3)

. The age of the layer is P11 (Middle Eocene). This

facies is thinning to the northwest. The source of sediment was derived from the southeast entrance

into the basin, and then spread towards the

northwest. Above this facies primary volcanic

products were deposited, such as lava and

pyroclastic rocks. In some places pillow lava structures are found (Figure 4). In the East of the

area of research a polymict breccia lithology is

developed with tuff fragments, clay and sand,

probably in a tuff matrix.

Figure 2. Compilation of Southern Mountains regional stratigraphy from previous authors.

Focus for this study is the lower part of the Kebo Formation, here concluded to range from P14 -

N5 (Middle Eocene - Early Miocene).

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Significant geological structures developed in the

study area, with several major faults. The Kalinampu-Mojosari fault is a strike slip with a

vertical component. The fault plane was observed

in the Kalinampu river cliffs, where the fault plane

parallels the direction of the river. Strike and dip of

the Kalinampu fault plane are N70°E/77° or

trending NE-SW. The lineament is terminated by

another NW-SE trending lineament. Other faults include the Mojosari reverse fault and the

Sumberan normal fault. The distribution of

formations and faults in the study area is shown

on the geological map of Figure 5.

Figure 3. Outcrop of dark volcanic breccia (A) underlain by white micrite

mudrock facies (B).

Figure 4. Outcrop of Nampurejo Lava with

pillow structures (A).

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Figure 5. Geological map of study area.

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METHODOLOGY

Three stratigraphic sections were measured in the study area to determine the stratigraphic

succession of rocks in the field (Appendices 1-3).

The measurement technique included the use of a

Jacob stick, so there is no need for thickness

corrections. Paleontological samples were taken from layers of fine-grained rocks and some of the

coarser-grained rocks, and then sieved to obtain

the small foraminifera fossils. The standard

sampling interval for paleontology was 1 meter.

However, because the foraminifera were most

abundant in the lower part of the section, paleontological sampling was maximized at the

base of the measured section. Petrographic

samples were taken to represent each of the facies

in study area.

BIOZONATION RESULTS

From the study of the distribution of foraminifera

in each measured section, followed by correlations, we identified 12 zones of planktonic foraminifera

as defined by Bolli et al. (1985) in the Kalinampu-

Sendangrejo section (Appendix 1), 7 zones in the

Sumberan-Mojosari section (Appendix 2) and one

zone in the Mranggen-Dukuh section (Appendix 3). The composite section in the study area contains

12 zones:

Zone 1 (Middle Eocene Zone-P11)

This zone is only found in the Kalinampu-

Senangrejo section. The lower limit of this zone is unknown while the upper boundary of this zone is

characterized by the appearance of Globigerinatheka subconglobata curryi. Planktonic

foraminifera species found in this zone include Globigerinatheka mexicana, Globigerinatheka index, Globigerinatheka subconglobata, Planorotalites pseudoscitula, Pseudohastigerina danvillensis, Truncorotaloides rohri and Turborotalia cerroazulensis.


This zone is found only in the Kalinampu-Sendangrejo section and is marked by Morozovella lehneri range zone, characterized by the presence

of Morozovella lehneri. Other fossils in this zone

include Acarinina spinuloinflata, Globigerina eocaena, Globigerinatheka mexicana kugleri, Globorotaloides carcoselleensis, Hastigerina cf. bolivariana, T. cerroazulensis possagnoensis - pomeroli transition, T. cerroazulensis and T. cerroazulensis pomeroli.


This zone is found only in the Kalinampu-

Sendangrejo section. The base of the zone is defined by the last occurrence of Globigerinatheka kugleri mexicana and the top by the last

occurrence of Globigerinatheka subconglobata.

Other fossils found in this zone includes Acarinina spinuloinflata, Catapsydrax dissimillis, Globigerina

eocaena, G. medizzani, Globigerinatheka subconglobata luterbacheri, Globorotaloides possagnoensis, Planorotalites pseudoscitula,

Pseudohastigerina danvillensis, Turborotalia cerroazulensis possagnoensis pomeroli transition, Truncatulinoides rohri, Turborotalia centralis and T. cerroazulensis pomeroli.


This zone is found in the Kalinampu-Sendangrejo

and Sumberan-Mojosari sections. In the Kalinampu-Sendangrejo section, the base of the

zone is marked by the first occurrence of Globigerina medizzani, the top by the last

occurrence of Planorotalites pseudoscitula. Other

foraminifera in this zone include Acarinina rugosoaculatea, A. spinuloinflata, Catapsydrax

dissimillis, Globigerina cryptomphala, G. eocaena, G. senni, G. mexicana, Globoborotaloides carcoselleensis, Pseudohastigerina danvilensis, T. cerroazulensis possagnoensis - pomeroli transition,

Turborotalia cerroazulensis pomeroli and

Truncorotaloides rohri. In the Sumberan-Mojosari

section, zone P14 is marked by a partial Globorotaloides carcoseleensis zone. The lower part

at this zone is unknown and the upper initiated by the first occurrence of Globorotaloides carcoseleensis. Foraminifera in this zone include

Catapsydrax dissimilis, Globigerinatheka subconglobata luterbacheri and Turborotalia cerroazulensis pomeroli.

Zone 5 (Late Eocene Zone-P15/16)

This zone was found in two sections. In the

Kalinampu-Sendangrejo section the base of the

zone is marked by the last occurrence of Globigerina sennii, the top by the first occurrence

of Globigerinatheka mexicana. Other fossils found

in this zone include Acarinina spinuloinflata, Globigerina cryptomphala, G. medizzani, Globigerinatheka index tropicalis, Globorotaloides carcoselleensis, Pseudohastigerina danvillensis, Turborotalia cerroazulensis possagnoensis - pomeroli transition and T. cerroazulensis pomeroli.

In the Sumberan- Mojosari section, this zone is bound by the first occurrence of Globigerinatheka subconglobata luterbacheri and ends with the first

occurrence of Globigerina praeturrilina. Other

species in this zone are Catapsydrax dissimilis, Globigerina ampliapertura, G. cryptomphala, G. eupertura, G. hagni, G. lozanoi, G. praebulloides occlusa, G. praeturrilina, G. tripartita and

Turborotalia cerroazulensis pomeroli. Reworked

fossils from erosion of older rocks include Globigerina corpulenta, G. sennii and Turborotalia griffinae.

Zone 6 (Late Eocene Zone-P16/17)

This zone is found in two sections. In the

Kalinampu-Sendangrejo section, the base of this zone is defined by the last occurrence of T. cerroazulensis possagnoensis - T. pomeroli

transition, while the top is marked by the first occurrence of Globigerina yeguaensis.

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Other fossils are Globigerina eocaena, G. hagni and

G. pseudovenezuelana. The most abundant species

is T. cerroazulensis pomeroli. In the Sumberan-

Mojosari section, the base of the zone is the first occurrence of Globigerina tripartita and it ends

with the first occurrence of Turborotalia cerroazulensis cunialensis. Other species present

are Catapsydrax dissimilis, Globigerina ampliapertura, G. praebulloides leroyi, G. sellii and

G. subconglobata luterbacheri.

Zone 7 (Early Oligocene Zone-P18/19)

This zone is also found in two sections. In

Kalinampu-Sendangrejo, the base of this zone is marked by the first occurrence of Globigerina yeguensis and the top by the first occurrence of

Globorotalia opima nana. Other fossils in this zone

are Catapsydrax dissimillis, Globigerina eupertura, G. ampliapertura, G. ciperoensis anguliofficinalis, G. cryptomphala, G. eocaena, G. ouachitaensis, G. praebulloides and G. praebulloides leroyi. The most abundant foram species is Globigerina yeguensis.

In the Sumberan-Mojosari section, this zone is defined by the last occurrence of Globigerina sellii at the base and the last occurrence of Globigerina tapurensis at the top. Other species in this zone

are Catapsydrax dissimilis, Globigerina ampliapertura, T. cerroazulensis pomeroli - cerroazulensis transition and Turborotalia centralis.

Reworked fossils include Globigerina hagni and G. subconglobata. The Globigerina ampliapertura

partial range zone is similar to P19. This zone is

unknown for the lower boundary and the top is marked by the last occurrence of Globigerina ampliapertura. Many fossils were found reworked:

Globigerina hagni, Turborotalia cerroazulensis cocoaensis, Globigerina lozanoi, Globorotaloides carcoselleensis, Hankenina alabamensis and

Turborotalia centralis.

Zone 8 (Middle Oligocene Zone-P20/N1)

In the Kalinampu-Sendangrejo section, the lower

part of this zone is marked by the first occurrence of Globigerina ampliapertura, the upper part by the

first occurrence of Globorotalia venezuelana. Other

fossils found in this zone are Globigerina yeguensis

and Globorotalia opima nana. The most abundant

species is Globigerina ampliapertura. In the

Sumberan-Mojosari section, this zone is between the last occurrence of Globigerina ampliapertura at

the base and the first occurrence of Globigerina yeguensis at the top. Other species in this zone are

Catapsydrax dissimilis, Globigerina ciperoensis anguliofficinalis, G. praebulloides leroyi and

Globorotalia opima nana. Reworked Eocene fossils

include Globigerapsis index, Globigerina pseudovenezuelana, Globorotalia ehrenbergi, Orbulinoides beckmanni and Planorotalites

pseudoscitula.

Zone 9 (Middle Oligocene Zone-P21/N2)

In the Kalinampu-Sendangrejo section, this zone is characterized by the range of Globorotalia opima opima. Other foraminifera in this zone are

Globigerina ampliapertura, G. tripartita, G.

venezuelana and G. yeguensis. In the Sumberan-

Mojosari section, this zone is also characterized by the Globorotalia opima nana range zone. Other

species present are Globigerina praebulloides occlusa and G. tripartita. There are many reworked

Eocene species: Globigerina medizzani, Globigerina praeturrilina, Globigerina sennii, Globorotaloides carcoselleensis, Hastigerina bolivariana, Morozovella lehneri, Planorotalites pseudoscitula

and P. renzi.

Zone 10 (Late Oligocene Zone-N3) The base of this zone is the last occurrence of Globorotalia opima opima and the top is the first

occurrence of Globigerina ciperoensis angulisuturalis. Other fossils found in this zone are

Catapsydrax dissimilis, Globigerina praebulloides

leroyi, G. praebulloides occlusa, Globigerinoides primordius, Globorotalia mayeri and Globorotalia opima opima transition to nana. The dominant

species is Globigerina angulisuturalis. In the

Sumberan-Mojosari section, this zone was not

found, but on the track Mranggen-Hamlet sample DKH12 contained Globigerina binaiensis,

Globigerina opima nana-opima transition and

Globorotalia mayeri and Globigerina tripartita. This

suggests most this section is within the zone N3

age range.

Zone 11 (Early Miocene Zone-N4)

This zone is only found in the Kalinampu-

Sendangrejo section and is marked by the range of Globigerinoides primordius. Other fossils found in

this zone are Catapsydrax dissimilis, Globigerina binaiensis, G. angulisuturalis, G. ciperoensis, G. praebulloides leroyi, G. praebulloides occlusa, G. pseudovenezuelana, G. tripartita, G. venezuelana, G. yeguensis and Globorotalia opima nana - opima opima transition.

Zone 12 (Early Miocene Zone-N5) This zone is found only in the Kalinampu-

Sendangrejo section and is between the first occurrence of Globigerina binaiensis at the base

and the last occurrence of Globigerina venezuelana

at the top. In the zonation of Blow (1969) this

corresponds to zone N5. Other fossils in this zone are Globigerina praebulloides leroyi, G. praebulloides occlusa, G. sellii, G. tripartita, G. venezuelana, G. yeguensis and the Globorotalia opima opima - G. opima nana transition.

DEPOSITIONAL ENVIRONMENTS

Interpretations of depositional environments were

made by using small benthic foraminifera.

Depositional environments of the Eocene - Early Miocene in this region are all deep, open marine

and range from lower bathyal to upper bathyal.

The Kalinampu-Sendangrejo section located in the

southern study area is in the areas of deepest

marine facies, as shown by the benthic foraminifera Bolivina robusta, Cibicides robertsonianus, Globulina minuta,

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Gyroidina broekhiana, Gyroidinoides soldanii, Lenticulina edinata, Neoponides magnitifer, Nodogenerina virgule, Nodosaria flintii, Oridorsalis

umbonata, Pseudoglandulina comatula, Uvigerina auberiana and Uvigerina schwageri. This

foraminifera association suggests sedimentation in

top of the lower bathyal zone.

The Sumberan-Mojosari section, located NE of the

Kalinampu-Sendangrejo section, shows a shallower paleoenvironment than the Kalinampu-

Sendangrejo section. Benthic foraminifera identified include Amphicoryna sp., Neoponides margaritifer, Planularia siddalliana and

Praemassillina arenaria. This foraminiferal

association suggests the sediments were deposited

near the top of the upper bathyal zone. The Late Oligocene in the Mranggen-Dukuh section in the

northern part of the study area is the shallowest.

The presence of the benthic foram species Globulina inaequalis suggests the depositional

environment is outer neritic.

DISCUSSION

The presence of Eocene planktonic foraminifera from the basal Kebo Formation in this part of the

Southern Mountains has not been documented

before, although many of the species mentioned in

Sumarso and Ismoyowati (1975) from the

Wungkal-Gamping Formation of the Jiwo Hills and

by Siregar and Harsono (1981) and Lunt and Sugiatno (2003) from the Nanggulan area are also

found in our study area.

The contact between the Eocene pillow lavas with

sedimentary rocks can clearly be seen in the Sumberan-Mojosari section, where sediments

above the pillow lava at Sumberan are correlated

with zone P14 (Middle Eocene), based on the presence of Truncorotaloides rohri. The Nampurejo-

Kalinampu pillow lava is located in the village and

does not have clear boundaries with sediments sampled in the Kalinampu-Sendangrejo section. It

is assumed to be the same as the Sumberan pillow

lava which is older than P14. The separation of the

pillow lava outcrops between Nampurejo and

Sumberan is assumed to be due to lateral shift by

the major shear fault in the area (Figure 5).

Surono (2009) mentioned that the Nampurejo

pillow lava is the base of the volcanoclastic Kebo-

Butak Formation sediments. From the biozonation

data obtained above, the Sumberan and Nampurejo pillow lavas are older than zones

Middle Eocene zones P12-P14. In the Kalinampu-

Sendangrejo section, we found sediments below

the pillow lava that indicate Middle Eocene planktonic foram zone P11 (Globigerinatheka subconglobata curryi zone). It is possible that the

Nampurejo pillow lava should not be viewed as the

base of the Kebo Formation sediments. The lava

flow may only represent one or more relatively

limited episode(s) of submarine basaltic volcanism

in a time characterized mainly by continuous Middle Eocene- Early Oligocene deep marine

pelagic sedimentation, and not necessarily the

onset of voluminous arc volcanics that characterize

the younger, Late Oligocene-earliest Miocene part

of the Kebo-Butak, Semilir and Nglanggran

sections. An alternative lithostratigraphic interpretation is to view the Middle Eocene-Early

Oligocene section in the study area as the deep

water equivalent of the Wungkal-Gamping Beds of

earlier authors (Figure 2). However, we do suggest

that starting from zone P12 (Middle Eocene; with the presence of Morozovella lehneri, Figure 6.3) the

stratigraphy in this area of the Southern

Mountains of Central Java is dominated by

volcanic material.

CONCLUSION

Biozonation analyses of planktonic and small

benthic foraminifera in three measured sections show that sediments in the study area were

deposited since zone P11 (Middle Eocene). It

demonstrates that the Middle-Late Eocene in this

part of the Southern Mountains of Central Java

was deposited in a deep, open marine facies,

deeper than age-equivalent sediments in the Nanggulan area further west.

Starting from zone P12 sediments are dominated

by volcanics, although the peak of volcanism, with

the highest sedimentation rates, appears to be around zone N3 (Late Oligocene). This proves that

volcanism in the Southern Mountains of Central

Java started much earlier than previously

suspected.

Appendix 1. Middle Eocene-Early Miocene biozonation of the Kalinampu-Sendangrejo section. This is the most complete of the three section studied. Total thickness is 390m, half of which is

interpreted as Zone N3 (Late Oligocene; Kebo-Butak volcanoclastics). Thickness of the Eocene basal part of the section (P11-P17) is 60m. Appendix 2. Late Eocene-Oligocene biozonation of the Sumberan-Mojosari Section. Total thickness 285m. Base of section is the Sumberan pillow lava, which is overlain by marl of Middle Eocene (zone P14) age. Most of the Middle Eocene- Early Oligocene section is composed of volcanoclastics.

Appendix 3. Late Oligocene biozonation of the Mranggen-Dukuh section. Total thickness 183m, entirely composed of thick volcanoclastic fining-

upward beds. Most samples from mudstone interbeds are barren, except for Late Oligocene (zone N3) planktonic foraminifera near top.

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REFERENCES Blow, W.H., 1969. Late Middle Eocene to Recent

planktonic foraminiferal biostratigraphy. Proc.

First Int. Conf. Planktonic Microfossils,

Geneva 1967, 1, Brill, Leiden, p. 199-422.

Bolli, H.M., Saunders, J.B. and Perch-Nielsen, K.,

1985. Plankton Stratigraphy. Cambridge University Press, London, 1032p.

Bothe, A.C.D., 1929. The geology of the Hills near

Djiwo and the Southern Range. 4th Pacific

Science Congress, Bandung, 23 p. Hartono, H.M.S., 1969. Globigerina marls and their

planktonic foraminifera from the Eocene of Nanggulan, Central Java. Contr. Cushman

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Kadar, D., 1986. Neogene planktonic foraminiferal

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area, Indonesia. Geol. Res. Dev. Centre, Bandung, Spec. Publ. 5, p. 1-83.

Lunt, P. and Sugiatno, H., 2003. A review of the

Eocene and Oligocene in the Nanggulan area, South Central Java, 34p. (Unpublished Report)

Siregar, P. and Harsono P., 1981. Stratigraphy and planktonic foraminifera of the Eocene-

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Paleont. Ser. 1, p. 9-28.

Sumarso and Ismoyowati, T., 1975. Contribution to the stratigraphy of the Djiwo Hills and their

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4th Ann. Conv. Indon. Petrol. Assoc. (IPA) p.

19-26, Jakarta.

Sumosusastro, S., 1956. A contribution to the

stratigraphy of Eastern Djiwo Hill and the Southern Range in Central Java. Indonesian

Journal of Natural Science 112, 115-134.

Surono, 2008. Litostratigrafi dan sedimentasi

Formasi Kebo dan Formasi Butak di

Pegunungan Baturagung, Jawa Tengah Bagian Selatan. Jurnal Geologi Indonesia 3, 4,

p. 183-193.

Surono, 2009. Litostratigrafi Pegunungan Selatan

bagian Timur Daerah Istimewa Yogyakarta

dan Jawa Tengah. Jurnal Sumber Daya Geol.

19, 3, p. 209-221. Toha. B., Purtyasti, R.D., Sriyono, Soetoto,

Wartono R., Subagyo P., 1994. Geologi daerah

Pegunungan Selatan, Suatu Kontribusi,

Fakultas Teknik, Universitas Gadjah Mada.

Van Bemmelen, R.W., 1949. The Geology of Indonesia, Vol. 1 A, Government Printing

Office, Nijhoff, The Hague, 732p.

Figure 6. Eocene planktonic foraminifera from the Kalinampu area. 1. Truncorotaloides rohri, 2.

Turborotalia cerroazulensis pomeroli, 3. Morozovella lehneri, 4. Globigerinatheka subconglobata curryi, 5. Globigerinatheka cryptomphala, 6. Turborotalia cerroazulensis cunialensis.

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APPENDIX 1

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APPENDIX 2

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APPENDIX 3

planktonic foraminifera biozonation of the middle eocene

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